The present invention relates to a crystalline silicate reforming catalyst and a process using the catalyst. More specifically, a combination of process conditions and improvements in the catalyst result in a low fouling rate.
Catalytic reforming is well known in the petroleum industry and refers to the treatment of naphtha fractions to improve the octane rating by the production of aromatics. The more important hydrocarbon reactions occurring during reforming include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, and dehydrocyclization of acyclic hydrocarbons to aromatics. A number of other reactions also occur, including the following: dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butane. Hydrocracking reactions are to be particularly minimized during reforming as they decrease the yield of gasoline boiling products and hydrogen.
Because of the demand for high octane gasoline for use as motor fuels, etc., extensive research is being devoted to the development of improved reforming catalysts and catalytic reforming processes. Catalysts for successful reforming processes must possess good selectivity. That is, they must be able to produce high yields of liquid products in the gasoline boiling range which contain large concentrations of high octane aromatic hydrocarbons and low concentrations of light gaseous hydrocarbons. Also, the catalysts should possess good activity in order that the temperature required to produce a certain quality product need not be too high. It is also necessary that catalysts either possess good stability in order that the activity and selectivity characteristics can be retained during prolonged periods of operation, or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Reforming catalysts are usually composed of a highly dispersed transition metal(s) on a metal oxide support. Typically, the transition metal is a noble metal, most notably platinum. However, there are numerous metal oxide supports. Examples are: silica, alumina, and a plethora of natural and man-made zeolites. Silicalite is one of those zeolites.
Silicalite is an intermediate pore zeolite and has a high silica:alumina (SiO.sub.2 :AlO.sub.3) ratio. Examples of its methods of manufacture can be shown in: Dwyer et al, U.S. Pat. No. 3,941,871, issued Mar. 2, 1976 and U.S. Pat. No. 4,441,991, issued Apr. 10, 1984; and Derouane et al, EPO Application No. 186,479, published Feb. 7, 1986, all of which are incorporated by reference in their entirety.
Dwyer et al suggest that a platinum-loaded silicalite catalyst can be used in reforming hydrocarbons, as well as other types of reactions. The process conditions in Dwyer et al are listed as: a temperature between 700.degree. F. and 1000.degree. F.; a pressure between 100 psig and 1000 psig (preferably 200-700 psig); a liquid hourly space velocity (LHSV) between 0.1 and 10 (preferably between 0.5 and 4); and a hydrogen to hydrocarbon (H.sub.2 /HC) mole ratio between 1 and 20 (preferably 4 and 12). Detz et al, U.S. Pat. No. 4,347,394, issued Aug. 31, 1982, disclose a process for selectively producing benzene using a catalyst having platinum on an intermediate pore zeolite which is substantially free of acidity (such as silicalite). The process conditions can be: temperatures greater than 480.degree. C. (more preferably at relatively high temperatures, such as between 510.degree. C. and 595.degree. C.); pressures of between atmospheric and 10 bar; an LHSV between 0.1 to 15; and hydrogen may or may not be added. P. Jacobs et al, "Comparison of Acid to Metal Catalyzed Conversion of N-decane and Cyclodecane on ZSM-5 and Faujasite-type Zeolites", J. Mol. Cat., 27, 11 (1984) show reacting a platinum silicalite catalyst with the test compounds n-decane and cyclodecane.
Reforming at low pressure in the absence of added hydrogen produces a relatively high liquid yield of relatively high octane reformate. Unfortunately, conventional catalysts foul quickly at these conditions which makes this operation impractical. Accordingly, the need has arisen for a reforming catalyst which has an acceptable run length under the conditions noted above.